1. Field of the Invention
The present invention relates to a voltage regulator, and more specifically, to a protection circuit with low current consumption whose operation is stopped in the case of light load.
2. Description of the Related Art
FIG. 5 is a circuit diagram for illustrating a related-art voltage regulator including a protection circuit.
The related-art voltage regulator includes a reference voltage circuit 101, an error amplifier 102, a PMOS transistor 106, resistors 107 and 108, a PMOS transistor 104, a constant current circuit 105, a resistor 111, a capacitor 112, a protection circuit 103, a constant current circuit 113 for the protection circuit 103, a VDD terminal 109, a VSS terminal 100, and an output terminal 110.
The PMOS transistor 104 and the constant current circuit 105 form an output current detection circuit configured to detect output current. When a heavy load is connected to the output terminal 110 and output current is thus large, the output current detection circuit outputs a detection signal. When the detection signal is output, constant current is caused to flow through the constant current circuit 113 to turn on the protection circuit 103. Then, the protection circuit 103 outputs a predetermined signal corresponding to the detection signal. When a light load is connected to the output terminal 110 and the output current is thus small, the output current detection circuit prevents the current from flowing through the constant current circuit 113 to turn off the protection circuit 103. Consequently, the voltage regulator consumes a small amount of current in the case of light load.
The resistor 111 and the capacitor 112 form a low-pass filter and prevent the protection circuit 103 from being erroneously operated when fluctuation in power supply voltage is large.
In the related-art voltage regulator including the protection circuit, in the case of light load, the current is prevented from flowing through the constant current circuit 113 to stop the operation of the protection circuit 103, and hence there is a problem in that the protection circuit 103 is repeatedly controlled to be on and off. The control of turning off the protection circuit 103 in the case of light load can be delayed with the low-pass filter of the resistor 111 and the capacitor 112. Thus, the above-mentioned repetitive operation can be avoided by changing, while the control is being delayed, the logic of an output of the protection circuit 103 to a logic that cancels an OFF state of the PMOS transistor 106.
However, when a load is suddenly changed from a light load to a heavy load and the operation of the protection circuit 103 thus needs to be started quickly, delay time due to the low-pass filter is further increased by a period of time required for starting activation of the constant current circuit 113. Consequently, when a load is suddenly changed from a light load to a heavy load, the start of the operation of the protection circuit 103 is delayed by the activation time of the constant current circuit 113.
The related-art voltage regulator including the protection circuit that only uses the low-pass filter has trade-off between solving the problem and the delay in start of the operation of the protection circuit 103, and hence no fundamental solution has been provided.